Doubling is a major quality control concern in the operation of multistage rotary machinery such as printing presses. Doubling is a register error among different printing units, which occurs whenever the impression on a printing cylinder blanket does not accurately line-up with the previous image remaining on the web. As the name suggests, doubling appears in a printed image as two impressions (i.e. the first on-register and the second off-register) causing the printed image to appear blurred, heavier, or fuller, than it should.
Doubling is typically caused by rotation non-synchronization of the printing cylinders among different units comprising the press. These rotation errors are induced by imperfections in the transmission systems within the units or the transmission links connecting different units. These imperfections may result from vibration, gear damage, bearing fault, excessive run-out, misalignment, or component wear.
For presses with web support rollers between units, imperfections or damage in roller bearings increase the roller rotation friction and induce variation in the web tension between different rollers. That web tension variation can cause doubling or smearing of the printed images, or even tear in the web.
Several methods have been proposed for detecting or preventing doubling problems in multistage rotary machinery such as printing presses. For example, U.S. Pat. No. 5,865,120 to Gross discloses a diagnostic system to determine the wear and damage of components in a printing press. The signals from different units are collected periodically and are processed using Fast Fourier Transform (FFT) techniques. The spectral components are correlated to each mechanical element of the press. By comparing the spectra with those of stored press-idiosyncratic reference signals, the changes between the two are applied for the diagnosis of the given rotary components. Deviations form a regular pattern may indicate an unhealthy pattern. If these deviations change over time and do not repeat, then they are considered to be the result of component wear or damage.
In U.S. Pat. No. 5,615,609 to Hill, et al., a system for controlling the registration of a multi-stage printing press for corrugated board materials is disclosed. Each printing unit has an AC drive motor, a controller, and a pulse signal generator. The controller is configured to receive the output pulses from a master encoder and follower encoders, process these output pulses to produce control commands, and transmit these control commands to the follower AC driving device so that the speeds of the follower AC motors are adjusted relative to the master AC motor to maintain rotation synchronization between the master and follower AC motors.
In U.S. Pat. No. 6,244,174 to Sirowitzki, et al., a diagnostic system for a sheet-fed offset printing machine is disclosed. A control system sets up a database recording the print jobs that are already completed. Then, a maintenance and inspection decision is made based on signals from different transducers in conjunction with recorded historic information in the database. For example, bearing wear level is detected with the help of temperature values from a temperature sensor placed in proximity to the bearing of interest, in conjunction with further production job information and recorded historic database information including printing material grade, type, number of colors to be printed, and printing pressure adjustments.
In U.S. Pat. No. 5,794,529 to Dawley, et al., a compliant drive system for printing presses is disclosed. This compliant drive system consists of a gear train and a circumferentially spaced spring coupling mechanism. The gear train meshes with the gears driving a plate cylinder, an ink vibrator and a water vibrator in a dampener, respectively. With the help of the spring coupling mechanism, the transmission of high frequency forces, vibrations and shocks from the ink and water vibrators, in the opposite direction, can be prevented. In this way, the occurrence of doubling problems is reduced.
Meshing inaccuracy of the gear train driving plate and blanket cylinders is one of the main causes of the doubling problem in printing presses. For example, U.S. Pat. No. 5,813,335 to Burke, et al. discloses an apparatus for preventing backlash between the gears driving the plate and blanket cylinders in a lithographic rotary printing press. This apparatus includes a supplementary gear, a support system, and a leaf spring assembly. The leaf spring assembly applies a force to the support element that biases the supplementary gear to make one gear in the gear train rotate in the opposite direction in order to offset the backlash between the gears driving the plate and blanket cylinders.
U.S. Pat. Nos. 5,671,636 and 5,357,858 to Gagne, et al. and Guaraldi, et al., respectively, disclose two types of systems for preventing circumferential separation when the gears driving the blanket and plate cylinders rotate at very high speeds. These systems consist of special torque transmitting gears and torsion springs. Torsional forces are applied to the corresponding gears in opposite rotational directions to keep the meshing teeth in contact.
Thus, the doubling problem is typically caused by rotational non-synchronization among the corresponding rotary components (e.g. cylinders and rollers) in different printing press units, in other words, among the gears driving these rotary components. One problem in detecting and preventing doubling is the difficulty in quantitatively determining rotational dissimilarities in real-time among the rotary components (e.g. gears) in different printing press units. A disadvantage of Gross, for example, is that it is difficult to detect gear damage using FFT techniques as spectral analysis usually fails to pick up the gear tooth faults. Since multistage rotary machines, such as printing presses, have complicated mechanical transmission systems, the spectra involved have too many spectral components to analyze effectively. Another disadvantage with existing systems is that they are not effective at determining gear run-out (which is related to factors such as bearing wear) or in using run-out information to diagnose doubling problems. Another disadvantage with existing systems such as that proposed by Hill, et al. is that if doubling is caused by factors such as vibration, it is difficult to offset the rotational non-synchronization among different units by the adjustment of the rotational speed of the driving motors in different units. A further disadvantage of existing systems (e.g. those disclosed by Gagne, et al., Guaraldi, et al., and Burke, et al.) is that they focus their techniques mainly on the gears driving the plate and blank cylinders. In fact, a fault with any rotary component of the printing press' transmission system may cause a doubling problem.
A need therefore exists for an improved method and system for diagnosing doubling in multistage printing presses. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.